1. Field of the Invention
The present invention relates to a zoom lens for projection, including four lens groups and being mounted on a projection display device, and a projection display device. Particularly, the present invention relates to a zoom lens for projection and a projection display device for magnifying and projecting a flux of light carrying video information from a light valve of a DMD (Digital Micro-mirror Device) display device onto a screen.
2. Description of Related Art
In recent years, a projector device (projection display device) using a DMD display device as a light valve has been noted.
The DMD is made by forming rectangular minute mirrors (mirror elements) of high reflectance, which can change the inclination in the range of 10 degrees or more in accordance with a video signal, on a silicon memory chip, using the CMOS semiconductor technology. The projector device using this DMD controls the reflecting direction of a light from a light source by changing the angle of the mirror elements, focusing only a desired reflected light onto the screen to enable a desired video to be projected.
In this DMD, for example, millions of mirrors or more are arranged in rows and columns on a substrate. Since these many mirrors can be digitally controlled independently of one another, each mirror corresponds to one pixel in the video.
Also, since it is unnecessary to polarize the illuminating light, unlike a liquid crystal display, there is less loss of light, whereby the DMD is excellent in the correctness of gradation representation.
In this manner, the DMD display device has many advantages, but there is a high degree of demand for an optical system mounted on the DMD display device to ensure the effectiveness of these advantages. Further, there is a growing demand for the projector device using the DMD that is convenient to carry or has good portability. It is required for the optical system to meet such demand for the portability.
By the way, most of the projector devices using the DMD adopt a so-called time division method in which an illumination system is constructed without a prism that is for the color synthesis or the separation of illuminating light or projected light and that is disposed on the reduction side of a projecting lens. In this case, since a space for disposing the prism is unnecessary, and the reduction side of the projecting lens is not required to be telecentric, it is demanded to further miniaturize the lenses by setting up the pupil on the reduction side at a position near the panel. In addition to the high image quality consistent with the resolution of the device, the wide angle of view and the zoom of high variable power ratio are further required from the viewpoint of establishment.
A zoom lens system capable of meeting the above requests to some extent was described in JP-A-2004-271668.
The techniques as described in JP-A-2004-271668 can meet the above various requests, including the wide angle of view and the high variable power ratio. However, to maintain the excellent aberration with the constitution as described in JP-A-2004-271668, it was required that a plurality of aspheric lenses were disposed, as will be clear from the examples.
Therefore, the processing and assembling loads of the optical system were increased, causing the manufacturing cost to rise.
JP-A-2006-78705 discloses a system configured to be telecentric on the reduction side, and fundamentally inconsistent with the invention in which the miniaturization is an important object. Also, a zoom lens for projection that is the three group zoom lens but and can be achieve object to the invention, and a projection display device are disclosed in JP-A-2007-271695.